Thermal protection systems (TPS) are widely used on reusable launch vehicles (RLV's) such as the Space Shuttle to provide a thermal shield against the very high temperatures of vehicle re-entry into the Earth's atmosphere. TPS may additionally be required on certain air vehicles such as hypersonic vehicles intended primarily for atmospheric flight. For example, TPS may be applied to portions of the air vehicle that are downstream of jet engine exhaust or rocket engine exhaust. Such air vehicles may include fixed structure or control surfaces that are located within the exhaust plume and which are therefore subjected to the extreme heat of such exhaust.
Future air and space vehicles to which TPS may be applied include crew exploration vehicles (CEV's) which may utilize modular architecture to transfer crew and cargo to the International Space Station and to destinations beyond. Other vehicles utilizing TPS may include vehicles having advanced air-breathing stages as well as supersonic air launch platforms combining air-breathing and rocket stages for transatmospheric or orbital missions.
As applied to RLV's, TPS must be capable of surviving extreme temperatures ranging from −300° F. on orbit to up to 3,000° F. upon re-entry into the Earth's atmosphere. Regarding its thermally insulative capabilities, TPS must be capable of maintaining the temperature of metallic or composite substructure below the temperature at which the mechanical properties of the substructure may begin to degrade. In addition to protecting the vehicle substructure against extreme temperatures, TPS must also accommodate flight-induced deflections of the vehicle substructure. For example, TPS must be capable of adjusting to relative movement of the substructure under high strain conditions of wing bending. In this regard, the attachment of TPS to the substructure must be of sufficient flexibility under high structural strain.
Prior art TPS for re-entry vehicles such as the Space Shuttle comprise a large number of insulative tiles formulated and/or sized for a variety of substructure material compositions. The tiles are positioned at strategic locations on the vehicle dependent upon the temperatures occurring at those locations. Current attachment technology for mounting TPS tiles to a substructure includes adhesive bonding. In one prior art attachment system, tiles may be first bonded to a strain isolation pad (SIP) which may be comprised of felt-like material. The SIP allows the tile to withstand flexing of the vehicle substructure under load. The SIP may also accommodate thermal growth of the tile and may compensate for acoustic excitation (i.e., vibration) during ascent to orbit. Following bonding to the tile, the SIP may be bonded to the substructure using a thin layer of silicone adhesive.
Although prior art systems for attaching TPS are generally effective for their intended purposes, they possess certain drawbacks which detract from their overall utility. More specifically, the installation of tiles using current attachment technology is extremely labor intensive and time-consuming. For example, high-temperature reusable surface insulation (HRSI) tiles are mounted on upper forward fuselage areas of the Space Shuttle and around certain portions of orbital maneuvering systems (OMS) and reaction control system (RCS) pods of the Shuttle.
The HRSI tiles are first bonded to an SIP which, in turn, is bonded to the Space Shuttle substructure using room temperature vulcanization (RTV) silicone adhesive. The RTV adhesive is applied to the substructure in thin layers of less than 0.010 inch. During the curing process, the tile/SIP may be forced against the substructure under pressure using a vacuum bag which is sealed against the substructure to enclose an area where the tiles are to be bonded. Replacement of the tile includes removal of old RTV followed by surface preparation of the substructure and then bonding and curing of a new tile element using the process described above. The total process is rather lengthy and can entail a significant amount of touch labor and vehicle down time.
A further drawback associated with current attachment technology for TPS tiles is that the RTV silicone bond line is limited to 500° F. continuous operation temperature. Unfortunately, this temperature limitation necessitates the use of extremely expensive polyimide structures directly under the TPS tiles in order to withstand the 500° F. operation temperature. An additional drawback associated with current attachment technology is related to the inability to inspect the silicone bond line after a tile is installed in order to verify bond quality. Also, current attachment technology of TPS tiles prevents quick access to underlying subsystems and structure for inspection, maintenance and servicing.
In order to improve the feasibility of hypersonic aircraft and future space vehicles, it is necessary to reduce the cost and time required to install TPS on such vehicles and to reduce the time required to inspect, remove, repair and replace tiles and underlying substructure. As can be seen, there exists a need in the art for an attachment system for TPS which facilitates rapid inspection of installed tiles and which allows for rapid access to underlying subsystems and structure for maintenance and inspection. Additionally, there exists a need in the art for an attachment system for TPS which allows for rapid replacement of damaged tiles and which facilitates relative movement between the tiles and substructure at operating temperatures in order to prevent failure of the connection between the tile and the substructure.
Furthermore, there exists a need in the art for an attachment system for TPS which allows for the use of low cost, low operating-temperature epoxy composite structures under the TPS in certain locations as opposed to the more expensive polyimide structures currently required to handle the 500° F. operating temperatures. In this regard, there exists a need in the art for an attachment technology for TPS which provides a relatively large cooling channel between the tile and the exterior of the substructure (i.e., air vehicle or space vehicle) skin in order to improve cooling capacity and permit operation at much higher temperatures. Finally, there exists a need in the art for an attachment system for TPS which reduces vehicle down time to repair or replace a damaged tile.